One way of manufacturing an optical preform is the Plasma Chemical Vapor Deposition (PCVD) process. The PCVD process is known, for example, from U.S. Pat. No. 4,314,833, which is incorporated by reference in its entirety. According to the disclosed process, one or more doped or undoped glass layers are deposited onto the interior of a substrate tube using low-pressure plasma in the glass substrate tube. After the glass layers have been deposited onto the interior of the glass substrate tube, the glass substrate tube is subsequently contracted by heating into a solid rod. In one embodiment, the solid rod may be externally provided with an additional amount of glass (i.e., by means of an external vapor deposition process), or by using one or more preformed glass tubes, thereby obtaining a composite preform. From this preform, one end is heated to produce optical fibers.
International Application No. WO 99/35304 (and its counterpart U.S. Pat. No. 6,260,510), which is incorporated by reference in its entirety, disclose microwaves from a microwave generator that are directed via a waveguide toward an applicator that surrounds a glass substrate tube. The applicator couples the high-frequency energy into the plasma. At one end of the substrate tube the doped or undoped reactive gases are supplied, after which a reaction takes place under the influence of the plasma, and doped or undoped glass layers are deposited onto the interior of the substrate tube. The other end of the substrate tube is connected to a vacuum pump so that a reduced pressure (e.g., between 5 and 50 mbar) is provided in the interior of the substrate. The applicator is moved reciprocally in the longitudinal direction of the substrate tube so that a thin glass layer is deposited onto the interior of the substrate tube with every stroke. The applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature of 900° C. to 1300° C. during the deposition process.
European Patent No. EP 1,550,640 (and its counterpart U.S. Publication No. 2005/0172902), which is commonly assigned with the present application and which is incorporated by reference in its entirety, disclose an apparatus for carrying out a PCVD process in which an annular choke having a specific length and width is positioned within the applicator such that the choke is centered around a cylindrical axis. The dimensions of the choke are selected to minimize the losses of high-frequency energy during the entire deposition process. This leads to more efficient energy consumption.
U.S. Pat. No. 6,901,775, which is incorporated by reference in its entirety, discloses an apparatus for internally coating a substrate tube by means of a PCVD process, from which substrate tube a preform is made. The gas delivery unit includes an insert, which is claimed to prevent a disturbance that induces a standing wave of a certain period and amplitude in the gas flow. According to this U.S. patent, a disturbance in the gas mixture flow can induce a standing wave of a certain period and amplitude in the gas mixture flow. This standing wave is responsible for a given deposition within the internal region of the substrate tube that is characterized by a non-uniform thickness along its axial direction. The non-uniformity in the thickness of the deposition translates to non-uniformity in the outer diameter of the corresponding collapsed preform and the resultant optical fiber.
These PCVD processes notwithstanding, there continues to be a need for an apparatus and method for achieving an optical fiber preform that possesses improved deposition uniformity along its length with respect to both refractive index and deposition thickness (i.e., cross-sectional area).